Tetrahydrobiopterin (BH4) has been found to inhibit the activation of phenylalanine hydroxylase that is brought about by limited proteolysis and by phosphorylation; phenylalanine overcomes this inhibition. Phenylalanine hydroxylase is in an activated state in livers from diabetic rats. Administration of insulin to these rats de-activates the enzyme. Rat liver phenylalanine hydroxylase has been shown to be composed of identical subunits.